Nuclear fuel containers under certain conditions are subject to leakage failures attributable to corrosion. For example, a destructive type of fuel container corrosion has been identified as fuel pellet-container or cladding interaction stress corrosion cracking, a phenomenon which is primarily induced or accelerated by abrupt or rapid reactor power increases.
Composite nuclear fuel containers or elements have been introduced in the art and used in commercial power generating, water cooled nuclear fission reactors to cope with the problem of stress corrosion cracking. Composite nuclear fuel containers comprise a generally conventional tubular container, constructed of a zirconium alloy, stainless steel, aluminum, or other suitable alloys of the art, provided with an internal lining which functions as a protective barrier. Such linings are composed of a metal having increased resistance to stress corrosion cracking, or other forms of destructive attack. The barrier linings of the art include a variety of metals and alloys, including zirconium metal of substantial purity, for example less than about 5000 parts per million impurities, copper, molybdenum, tungsten, rhenium, niobium and alloys thereof. Examples of such protective metal barrier linings for nuclear fuel tubular containers comprise U.S. Pat. Nos. 4,200,492, issued Apr. 29, 1980; No. 4,372,817, issued Feb. 8, 1983; No. 4,445,942, issued May 1, 1984; No. 4,679,540; issued Apr. 21, 1987; No. 4,942,016, issued Jul. 17, 1990; and No. 4,986,957, issued Jan. 22, 1991.
Typical composite fuel containers for water cooled nuclear reactors comprising a tubular container provided with a metal liner internal barrier metallurgically bonded on to its inner surface, are generally produced by inserting a section of hollow liner stock in close fitting intersurface contact within and throughout the length of a section of tube stock, then forming the metallurgical bond by conventional means. Several methods can be used for metallurgically bonding the tube section to the liner component, including explosive bonding, heating under compression loading to cause diffusion bonding and extrusion of the assembly. Examples of such methods for producing composite constructed nuclear fuel containers are given in U.S. Pat. Nos. 4,200,492, issued Apr. 29, 1980; No. 4,372,817, issued Feb. 8, 1983; and No. 4,390,497, issued Jun. 28, 1983.
This composite assembly of a section of tube stock with the liner stock inserted within the length thereof, is then subjected to a series of circumference reductions with each reduction accompanied by a following heat annealing to reduce any stresses introduced by the compression distortions of the diameter reduction.
In addition to the conventional annealing heat treatments for the purpose of relieving reduction compression induced stresses in the metal crystalline structure of the reduced composite tube and liner unit, it is a common practice to subject nuclear fuel containers to specific heat modifying treatments to enhance or optimize a critical property of the metal such as corrosion resistance or ductility for improving the durability of the container. For example, it is an established practice to heat treat zirconium metal and its alloys, or fuel containers containing same, up to a temperature above the alpha microcrystalline phase of the particular metal composition, or into the beta microcrystalline phase, followed by rapid cooling to impart corrosion resistance. Such recrystallization heat treatments are disclosed in detail in the art, note for example U.S. Pat. Nos. 2,894,866, issued Jul. 14, 1959; No. 4,238,251, issued Dec. 9, 1980; No. 4,390,497, issued Jun. 28, 1983; and No. 4,576,654, issued Mar. 18, 1986.
Heat treatment temperatures for producing the microcrystalline structure changes and accompanying property modifications disclosed in the literature typically depend upon the exact composition of the metal or alloy, and are essentially a unique condition for each different metal or combination of alloying ingredients. Accordingly if the temperature levels to achieve or optimize a particular characteristic in a specific metal composition is not provided in the literature, it can be determined empirically, note for example U.S. Pat. Nos. 2,894,866, issued Jul. 14, 1959; and No. 4,238,251, issued Dec. 9, 1980.
The disclosure and contents of all the aforesaid U.S. patents, and the references cited therein, are incorporated herein by reference.